DISTYLY, SELF-INCOMPATIBILITY, AND EVOLUTION IN MELOCHIA.

Abstract

The genus Melockia of the subfamily Hermannieae, Sterculiaceae, includes about 60 species which are widely distributed through the tropics, and, less extensively, through the temperate zone. An encounter with a distylic species, M. tomentosa L., on the desert island of Mona, suggested this study of selfand cross-incompatibility. The occurrence of distyly in this genus evidently has been overlooked by specialists in the field (Crowe, 1964; Pandey, 1957; Brewbaker, 1957) and is not noted in floras of the Caribbean area. Heterostyly is considered to be an advanced system by Crowe (1964), because it occurs in 20 advanced families of the angiosperms, but not in primitive families. Its widely scattered occurrence suggests that it has arisen separately on a number of occasions. One system of self-incompatibility has been described in the family Sterculiaceae. Knight and Rogers (1955) suggested that the system in Theobroma cacao L. is controlled by a series of incompatibility or S alleles, with or without dominance relations among them, and with control of phenotype of ovule and pollen expressed before meiosis. The site of the incompatibility reaction is the ovary. Incompatibility is due to the failure of fertilized ovules to develop (Knight and Rogers, 1955). Ovules are thus sterilized by fertilization, and cannot be stimulated by later compatible pollinations. The system is homomorphic, that is, not characterized by morphological variations of stigma, style, or pollen associated with the incompatibility phenotype. Cope (1958, 1962) found evidence that a portion (25, 50, or 100 per cent) of the ovules in the incompatibility pollinated ovary do not exhibit fusion of gametes, and that flowers absciss, even though gamete fusion occurs in other ovules as a result of compatible fertilization. Compatible matings are not characterized by such non-fusion ovules. These findings suggest a segregation of pollen types in some cases, and therefore a gametophytic or haploid determination of pollen phenotype. However, crossing reactions among parents and offspring indicate that sporophytic effects are also present. Pandey (1960) and Cope (1962) have recently interpreted the physiological and genetic basis of incompatibility in the genus. The Theobroma system is considered unique in the plant kingdom (Crowe, 1964) although other systems, still not thoroughly investigated, resemble it (Lilium, Freesia, Hemerocallis, Gasteria, and Narcissus). Its characteristics are sufficiently dissimilar from other systems to suggest to Crowe (1964) the separate evolution of this system from a primitive, hypothetical system underlying incompatibility in flowering plants. In addition to Theobroma, self-incompatibility has been found in the closely related genus Herrania (McKelvie, as reported by Fryxell, 1957). The details of the system have not been worked out. East (1940), in his survey of self-incompatibility in the flowering plants, did not report any other self-incompatible species in the family. Recently species of Dombeya have been found to be self-incompatible (Soderholm, personal communication, 1965). Because of the peculiarity of the Theobroma incompatibility system it is of great interest to encounter another system, distyly, in Melockia.